Years ago I had a 1983 Merc 150 that did that. I needed a temp fix for a fishing trip that I was leaving for. It was a slow leak so I added some TransX to the fluid and it stopped the leak. I had all intentions of fixing it later but completely forgot about it until after I sold the boat several years later. It never did start leaking again. I strongly suspect it was a seal that had got hard from sitting so long with no use. Most auto parts stores carry it. Its worth a try.